A public key blob (PUBLICKEYBLOB) contains the public key portion of a public/private key pair. Unlike simple key blobs, these are not encrypted.
When communicating with someone who is not using CryptoAPI, your application may need to build one of these key blobs manually so the other user's public keys can be imported into your CSP. In addition, it is not unheard-of for an application to display the value of a public key in plaintext form so the user can validate it by hand.
The format of public key blobs is fully documented in the Public Key Blob Format section.